1. Field of the Invention
This disclosure relates to luminescent layers suitable for light-emitting devices, such as laminated translucent and transparent ceramic elements and methods for making the same.
2. Description of the Related Art
Solid state light-emitting devices such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs) sometimes called organic electroluminescent devices (OELs), and inorganic electroluminescent device (IEL) have been widely utilized for various applications such as flat panel displays, indicators for various instruments, signboards, and ornamental illuminations, etc. As the emission efficiency of these light-emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. For these applications, white LED is one of the promising candidates and has attracted much attention.
Conventional white LED's are manufactured based on a combination of blue LED and yellow light-emitting YAG:Ce phosphor powder used as a wavelength-converting material dispersed in an encapsulant resin such as epoxy and silicone, as disclosed in U.S. Pat. No. 5,998,925 and U.S. Pat. No. 6,069,440. The wavelength-converting material is so disposed as to absorb some part of the blue LED light-emission and re-emit the light at a different wavelength as yellow or green-yellow light. The combination of the blue light from the LED and the green-yellow light from the phosphor results in perceived white light. A typical device structure is shown in FIGS. 1A and 1B. A submount 10 shown in FIG. 1A has a blue LED 11 mounted thereon, covered with a transparent matrix 13 in which YAG:Ce phosphor powder 12 is dispersed and encapsulated by a protective resin 15. As shown in FIG. 1B, the blue LED 11 is covered with a transparent matrix 13 in which YAG:Ce phosphor powder 12 is disposed. However, since the particle size of YAG:Ce phosphor powder utilized for this system is around 1-10 μm, the YAG:Ce powder 12 dispersed in the transparent matrix 13 can cause strong light scattering. As a result, as shown in FIG. 2, a considerable portion of both incident light 18 from the blue LED 11 and yellow light 19 emitted from the YAG:Ce powder ends up being backscattered and dissipated, causing a loss of white light emission.
One solution to this problem is to form a monolithic ceramic member as a wavelength-converting material. The ceramic member can be constituted by plural ceramic layers of single or multiple phosphors, or transparent layers. The transparent ceramic layers may be constituted by, for example, the same host material as the wavelength-converting material, but may be devoid of any dopant (U.S. Pat. No. 7,361,938). These laminated layers may also be in the form of luminescent ceramic cast tapes, which can be laminated and co-fired (U.S. Pat. No. 7,514,721 and U.S. Published Application No. 2009/0108507). However, since these laminated layers are generally formed from garnet powder of low IQE (Internal Quantum Efficiency) produced through solid state reaction or co-precipitation, the present inventors recognized that the resultant luminosity generated by these luminescent layers is poor even though the cost of manufacture is low. Phosphor nanoparticles produced by radio frequency thermal plasma treatment of liquid precursors showed high wavelength conversion efficiency (WO2008/112710) and very well controlled stoichiometry, but generally have high production costs. As a result, monolithic ceramic plates composed entirely of plasma nanoparticles would increase production costs.
In addition, high IQE nanoparticles, when formed into ceramics, do not necessarily result in ceramic layers characterized by high wavelength conversion efficiency (WCE). IQE is a measure of the conversion efficiency of the photons striking the emissive material converted into the photons emitted by the emissive material. WCE is a measure of the conversion efficiency of blue light converted into white light. Thus, in white light emitting devices, WCE is of particular importance.
The present inventors recognized that thin layers of phosphor ceramics with sufficiently high activator content having a thickness on the order of tens of microns/micrometers can reduce production costs significantly. Nevertheless, while being appropriate for color conversion, the thin phosphor layers are rendered fragile and difficult to handle. Thus, the present inventors recognized that there is a need for an effective way to enhance the light output from white LEDs while minimizing the backscattering loss without sacrificing luminance intensity of phosphor powder.